In the field of optical communication, a digital coherent communication system that combines a coherent detection scheme with digital signal processing, which dramatically increases frequency use efficiency, is attracting attention. The digital coherent communication system is known to be able to not only improve reception sensitivity but also compensate for waveform distortion of a transmission signal caused by chromatic dispersion and/or polarization mode dispersion in optical fiber transmission by receiving the transmission signal as a digital signal as compared to systems built using direct detection, and introduction thereof is being studied as a next-generation optical communication technology.
Digital coherent schemes represented by Non-Patent Document 1 and Non-Patent Document 2 adopt a method for compensating for quasi-static chromatic dispersion by means of a fixed digital filter (for example, the number of taps is 2048 for the dispersion of 20000 ps/nm and a signal of 28 Gbaud) and compensating for fluctuating polarization mode dispersion by means of an adaptive filter with a small number of taps (for example, about 10 to 12 taps for the polarization mode dispersion of 50 ps) using a blind algorithm.